A typical projection system includes an arc-lamp source of bright white light, a method for separating the white light into red, green and blue spectral components, and a spatial light modulator (SLM, also called a light valve) for two dimensional spatial imaging each of the spectral components over time to create a color picture. The SLM performs the spatial and temporal modulation in response to an analog or digital video signal generated by a computer or another video source such as a TV tuner, VCR, HDTV broadcast, or DVD player. The SLM typically creates sequential images in red, green, and blue color planes using a color wheel although other color separation techniques exist such as cyan, yellow, magenta, and optionally white planes. The color planes may then be recombined optically and projected onto a screen, or they may be flashed temporally in sequence onto the screen at such a rate that a viewer perceives only a single image.
Most digital projectors compete based on the number of screen lumens that the projector places on the projection screen. Although the competition in the projector market is fierce, cost considerations as well as size constraints have limited the development of more efficient light sources.
There are several problems with existing light sources used in digital projectors. Many arc lamp sources have a visible light spectrum that is non-uniform in irradiance. This non-uniformity requires that systems in which color wheels are used to have color wheels with specific coating designs to filter out portions of the non-uniform spectrum that are unwanted.
For instance, with a mercury-arc lamp, there are two intense spikes in the spectrum from 520 nm (nanometers) to 600 nm. One intense spike occurs from 520 nm to 560 nm. The other spike occurs from 560 nm to 600 nm. This last spike has a very intense yellow line spectrum that makes it difficult to achieve a good balance color gamut. Accordingly, this last spike is conventionally filtered out by the color wheel. By filtering out light in this spectrum, there is a loss of lumens reaching the spatial light modulator and ultimately the screen.
For example, the designs of conventional color wheel coatings are chosen to adjust the slopes and overlaps of the green and red filters on the color wheel to achieve good color balance. Typically, a red portion of the color wheel is designed to allow light to pass with wavelengths of 600 nm to 800 nm. Thus, the spike at 560-600 nm is essentially and substantially eliminated by the color wheel. Accordingly, there is a corresponding reduction in brightness generated by the projector.
In summary, there exists a need to overcome the efficiency, non-uniformity in spectral irradiance and other problems associated with arc lamp bulbs, particularly mercury vapor types.